Page 419 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

374 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

with time. The ‘‘clogging front’’ is seen to move downward P(O 2 , atmosphere, Table H.1 or B.7) ¼ 0.209476
with time and is detected by the beginning of the linear part of (mol fraction O 2 )
the headloss versus distance plot. The illustration indicates that P(N 2 , atmosphere, Table H.1 or B.7) ¼ 0.78084 (mol
fraction N 2 )
‘‘terminal’’ headloss occurs at t 6 h. Valve ‘‘E’’ is a rate-of-
2. Atmospheric pressure at 1524 m (5000 ft) from
flow-controller and is opened only a slight amount at the start
Figure H.2 is, P(atm, 1524 m) 0.85 atm
of the run, that is, at t ¼ 0, but is opened fully at t ¼ 6 h. The
3. In terms of water pressure, 0.85 atm 10.33 m water=
total headloss available is distributed between the media-bed
atm 8.78 m water (absolute pressure).
and Valve E. In the illustration, the tailwater elevation is the 4. Absolute pressure at Tap D is,
same as the surface of the media-bed.
H(D-absolute) ¼ 8.78 m 2.0 m ¼ 6.78 m water
0.65 atm.
12.5.2.3 Negative Pressure 5. Saturation concentrations of pure gases at 1.0 atm
are as follows:
Figure 12.40b shows the same filter bed as seen in Figure
C(1.0 atm pure O 2 ,208C) ¼ 43.39 mg=L
12.40a but the tailwater is below the level of the media-bed
C(1.0 atm pure N 2 ,208C) ¼ 19.01 mg=L
surface. The HGL is shown only for terminal headloss;
6. Saturation concentrations of gases at their respective
for this condition, the HGL elevations are below the media partial pressures in the atmosphere at 1524 m
taps, for example, taps, B, C, D, and E. Therefore, negative elevation are as follows:
heads (or pressures, i.e., p ¼ gh) occur in the media at these C(O 2 ,208C, 0.85 atm absolute) ¼ 7.73 mg=L (i.e.,
respective tap elevations. The ‘‘head’’ (or pressure) at a given 43.39 0.209 0.85)
tap equals the difference between its HGL elevation and C(N 2 ,208C, 0.85 atm absolute) ¼ 12.62 mg=L (i.e.,
its tap elevation. Consider Tap ‘‘D,’’ for example: let Elev 19.01 0.781 0.85)
(HGL-D) ¼ 8.00 m and Elev(tap-at-D) ¼ 10.00 m; then 7. Saturation concentrations of gases at elevation
2.0 m water pressure at 1524 m elevation, which
Head(D) ¼ 8.00 10.00 ¼ 2.00 m. In other words the pres-
is 0.65 atm absolute pressure, is their respective
sure within the filter bed at Tap D is negative, for example,
partial pressures in the atmosphere at 1524 m
2.00=10.33 ¼ 0.19 atm.
elevation are:
C(O 2 ,208C, 0.65 atm absolute) ¼ 5.91 mg=L (i.e.,
12.5.2.4 Air Binding 43.39 0.209 0.65)
Gas precipitation, when it occurs in filters, causes ‘‘air bind- C(N 2 , 1524 m, 208C, 0.65 atm absolute) ¼ 9.65
ing’’ (Fair and Geyer, 1961, p. 699; see also, Section mg=L (i.e., 19.01 0.781 0.65)
8. Therefore, since
12.5.2.3), which occurs when gas bubbles occupy volume
7.73 > 5.91 mg O 2 =L, oxygen gas will precipitate.
within the filter bed. In such a case, the headloss increases
12.62 > 9.65 mg N 2 =L, nitrogen gas will precipitate.
inordinately and at the same time causes higher than average
interstitial velocities. The effects of air binding may be
observed during backwash as ‘‘boils’’ of large air bubbles Discussion
breaking the water surface. Such precipitated air may disrupt A not uncommon design has been to locate the clear-well
a gravel support. below the filter bed bottom, with a pipe from the under-
Gas precipitation can be avoided by positioning the weir drains discharging into the clear-well. Thus when the
crest of the tailwater at the same level as the top of the media- rate-of-flow-controller valve is open all the way the
bed, or not too far below (see also Monk, 1987). This works HGL will drop below the media elevation (as illustrated
in Figure 12.40) which is, by definition, a negative pres-
unless the gas concentrations exceed what would exist at
sure, which may cause gas precipitation. As another
equilibrium with the atmospheric conditions at hand, that is,
issue, if the water is ‘‘supersaturated’’ on entering the
when ‘‘supersaturated.’’
filter bed, for example, due to air bubbles being entrained
in a pipeline that drops in elevation, or due to algae
photosynthesis, gas precipitation could occur even if the
Example 12.11 Evaluation of Whether Gas Will tailwater elevation is at the same level as the filter bed
Precipitate in Filter surface. To avoid gas precipitation, the gas must be
removed before entering the filter bed (see Chapter 18
and Appendix H).
Given
Let elevation of a filter be 1524 m (5000 ft). The tailwater
elevation is lower than the surface of the filter bed as
shown in Figure 12.40b. A piezometer tap ‘‘D’’ has a 12.5.3 BACKWASH
HGL level-2.0 below the level of the tap.
At the end of a filter run, the filter is backwashed to remove
Required the attached floc from the media grains. Traditional backwash
Determine whether gas precipitation will occur. involves bed fluidization. Ancillary steps may include sur-
Solution face-wash or air scour, or both (see also Logsdon, 2008,
1. Partial pressure of oxygen and nitrogen in atmos- pp. 115–145). An inadequate backwash is likely to result in
phere is as follows: ‘‘mudball’’ formation.